JEMS Talk - School of Physics and Astronomy

Transcript JEMS Talk - School of Physics and Astronomy

Electron [and ion] beam studies of magnetic nanostructures

John Chapman, Department of Physics & Astronomy, University of Glasgow

Synopsis

Domain wall structures Investigation by Lorentz microscopy Domain wall widths in soft films Magnetisation reversal processes in soft high-moment films Single layer films The effects of lamination – desirable and otherwise!

Magnetisation reversal processes in magnetic elements Vortices The role and elimination of metastable states Domain wall traps Notches (constrictions) in magnetic wires [Domain wall modification by ion irradiation]

Schematics of 180



domain wall structures

M + + + M M M M Schematic of Bloch wall M + + + M M + + + M Schematic of Neel wall M Schematic of cross-tie wall

Fresnel imaging mode

B O





Fresnel intensity divergent specimen object plane (Fresnel

)

diffraction plane image plane convergent x 3.8 Oe 15 m m 20 nm permalloy film; H parallel to hard axis

Differential phase contrast (DPC) imaging of a 180



domain wall in a soft magnetic film

probe-forming aperture

B o

scan coils specimen Direction of induction mapped 

de-scan coils post specimen lenses quadrant detector 5 m m

Experimental and theoretical domain wall widths in a 8 nm thick permalloy film

a A B b 300 250 200 150 100 300 250 200 150 100 50 0 0 50 200 400 600 800 1000 1200 1400 1600 0 0 200 400 600 800 1000 1200 1400 1600 180° wall profiles in 8 nm thick permalloy: (a) as the free layer of a spin-valve and (b) as an isolated layer Fitting function: tanh(2x/w) w (nm) SV 150 + 4 isolated layer 154 + 4 From Hubert (Phys. Stat. Sol.

, 699, 1969) w (nm) 157 300nm

Synopsis

Domain wall structures Investigation by Lorentz microscopy Domain wall widths in soft films

Magnetisation reversal processes in soft high-moment films Single layer films The effects of lamination – desirable and otherwise!

High-moment CoFe multilayer films

ML1 CoFe 50 nm NiFe 1 nm ML3 CoFe 10 nm NiFe 1 nm Al 2 O 3 1.5 nm ML2 CoFe 22.5 nm NiFe 1 nm Al 2 O 3 1.5 nm ML4 CoFe 10 nm NiFe 1 nm Al 2 O 3 1.25 nm Laminating does not significantly change the total moment of the film but it changes the magnetisation curve and can lead to lower noise in devices.

Easy and hard axis magnetisation reversals – ML1

CoFe 50 nm NiFe 20 15 10 5 0 -5 -10 -15 -20 -100 -80 -60 -40 -20 0

20 40 60 80 100

Easy Hard

H c  15 Oe 3 m m easy axis H a +25Oe 3 m m -13Oe -14Oe -15Oe -20Oe easy axis H a +41Oe +10Oe +3Oe -8Oe -10Oe

Easy and hard axis magnetisation reversals – ML2

CoFe 22.5 nm NiFe Al 2 O 3 20 15 10 5 0 -5 -10 -15 -20 -60 -40 -20 0

20 40 60

Easy Hard

H c  5.3 Oe 2 m m easy axis H a +45 Oe 2 m m +8 Oe -2 Oe -4 Oe -41 Oe easy axis H a +32 Oe +9 Oe -1 Oe -5 Oe -21 Oe

Easy and hard axis reversal behaviour

Easy axis Hard axis easy axis H a Small number of mobile domain walls easy axis H a Larger number of less mobile domain walls Domain wall

Néel walls in bilayer films

1 m m +1 Oe

+ +

Schematic representation of twin Néel walls

+ + -

Schematic representation of superimposed Néel walls

Easy and hard axis magnetisation reversals – ML3

CoFe 10 nm NiFe Al 2 O 3 0 -5 -10 -15 20 15 10 5 -20 -100 -80 -60 -40 -20 0

20 40 60 80 100

Easy Hard

H c  2.8 Oe 1 m m easy axis H a +31 Oe 1 m m +7 Oe 0 Oe -2 Oe -31 Oe easy axis H a +32 Oe +15 Oe 0 Oe -13 Oe -32 Oe

Easy and hard axis magnetisation reversals – ML3

CoFe 10 nm NiFe Al 2 O 3 0 -5 -10 -15 20 15 10 5 -20 -100 -80 -60 -40 -20 0

20 40 60 80 100

Easy Hard

H c  2.8 Oe 2 m m easy axis H a +33 Oe 2 m m +14 Oe +1 Oe -5 Oe -30 Oe easy axis H a +33 Oe +25 Oe +13 Oe 0 Oe -33 Oe

Easy and hard axis reversals of soft magnetic films

Easy axis – NiFeCuMo layer Hard axis – NiFeCuMo layer Easy axis – ML4 Field range for NiFeCuMo film: ±10 Oe Field range for ML4: ±60 Oe

Easy and hard axis magnetisation reversals – ML4

CoFe 10 nm NiFe 1 nm Al 2 O 3 1.25 nm 20 15 10 5 0 -5 -10 -15 -20 -100 -80 -60 -40 -20 0

20 40 60 80 100

Easy Hard

H c  3.4 Oe 3 m m easy axis H a -30 Oe 3 m m -11 Oe 0 Oe +2 Oe +28 Oe easy axis H a -30 Oe -11 Oe 0 Oe +14 Oe +28 Oe

Hard axis magnetisation process preserving 360° domain walls

easy axis H H = 0 Reduce H High H Corrugations collapse New 360 0 wall forms Small reverse H Wall disappears unstable so corrugates

Easy axis magnetisation process preserving 360° domain walls

M easy axis H small reversed H New 360 0 wall forms here after switch H=0 Wall disappears Provided there is something to pin the ends of the 360  walls, their behaviour under an applied field and high degree of stability is readily comprehensible.

The fact that walls form in particular locations suggests that their origin is closely related to the local microstructure of the laminated films.

Cross-sectional TEM images of ML1 and ML3 20nm

Growth direction

20nm

Growth direction

Summary of the behaviour of the high-moment CoFe multilayer films

• • • • 180 ○ domain walls with cross ties were observed in the single layer films with a seedlayer, consistent with their 50nm thickness.

Much improved magnetisation curves were found for the laminated films.

However, defect areas and 360 ○ domain walls were also frequently present in structures with many layers. The comparatively low contrast suggested they did not exist in all the layers in the multilayer stack.

The behaviour and resilience to annihilation of the 360 ○ domain walls requires strong pinning at the ends; normal TEM imaging reveals nothing unusual about the regions where the ends were located.

Cross sectional TEM revealed decreasing grain size but increasing roughness with increasing number of spacer layers. The former is the probable origin of the decreasing coercivity and the latter of the complex local inhomogeneous magnetisation distributions that form. Local fields >100 Oe are expected where the roughness is greatest.

Synopsis

Magnetisation reversal processes in magnetic elements Vortices The role and elimination of metastable states Domain wall traps Notches (constrictions) in magnetic wires

[Domain wall modification by ion irradiation]

Vortex structures: experiment and simulation

9 nm 50 nm 250 nm Distance nm 1 2

Metastable states in rectangular elements

1 μ m C-state

S-state C-state

On application of field C

S C

S-state

E C



E S

Flux Closure

Domain wall traps

Unlike simply shaped magnetic elements that to a zeroth order approximation are single domain structures, domain wall traps are (to the same approximation) two domain structures separated by a head-to-head domain wall.

Various geometries are possible for the domain wall packet separating the oppositely magnetised domains.

In the traps we have studied, magnetic vortices are found frequently.

Domain wall trap based on a compliant vortex domain wall structure

0 Oe

Dimensions of central section: 1000 x 200 nm 2 M

+H -15 Oe +25 Oe -40 Oe

250nm

Movement of domain wall packet in a trap

Field variation from 0 Oe to -40 Oe to +40 Oe and back several times Field variation from 0 Oe to -100 Oe and back

Reversing “domain wall packet” in a permalloy wire close to and at a constriction

Wire width: 500 nm; wire thickness: 20 nm

200 nm

Reversing “domain wall packets” in permalloy wires at constrictions of different geometry

200 nm

Thickness 20 nm 200 nm Thickness 30 nm

Wires with constrictions – the reversal process

Field range: -250 Oe to +250 Oe then back to –250 Oe

Wires with constrictions – the reversal process

0 Oe

- 116 Oe

= 500 nm d = 100, 150 nm l = 750 nm

- 174 Oe - 182 Oe - 230 Oe

Summary

• Magnetic vortices are found frequently in wall structures in small elements; their core is typically <10 nm in extent.

• Metastable domain configurations that occur in elements with high symmetry can be eliminated by lowering the element symmetry and/or by the introduction of notches leading to more reproducible switching behaviour.

• An alternative bi-state element is the domain wall trap; reproducible behaviour and lower switching fields can be obtained, but at the expense of a larger element area.

• Notches (constrictions) in wires also act as local pinning sites; the structure of head-to -head domain walls in their vicinity is rarely simple and differs from that in the uniform parts of the wire.

Acknowledgements

Stephen McVitie, Beverley Craig, Craig Brownlie, Aurelie Gentils, Damien McGrouther, Nils Wiese, Xiaoxi Liu, Chris Wilkinson (University of Glasgow) Alan Johnston, Denis O’Donnell (Seagate Technology) Bob McMichael, Bill Egelhoff (NIST)